The present invention relates to an exhaust gas recirculation device of an engine, which recirculates part of exhaust gas discharged from an exhaust passage provided inside a cylinder head of an engine body having plural cylinders arranged in line in a longitudinal direction to an intake system of the engine body.
Conventionally, it has been done to recirculate part of exhaust gas to an intake system and decrease the temperature of combustion gas with the recirculated EGR gas to reduce an emission of NOx. In general, a device in which part of the exhaust gas is recirculated to the intake system from an exhaust manifold which is connected to a cylinder head outside the cylinder head through an EGR passage provided outside the cylinder head is known as the above-described exhaust gas recirculation device to recirculate part of the exhaust gas, i.e., EGR gas, to the intake system. Further, as disclosed in Japanese Patent Laid-Open Publication No. 2002-285915, a device in which an EGR passage to recirculate the EGR gas is formed inside the cylinder head and the EGR gas is recirculated to the intake system through this EGR passage has been developed as well. This device may make the device compact overall.
In the device disclosed in the above-described patent document, the high-temperature EGR gas is directly recirculated to the intake system. Accordingly, according to this device, under a certain driving condition in which the combustion is unstable at an engine start, a low engine temperature or the like, the temperature of intake air is so increased that the combustion can be stabilized. Under the other condition, however, the intake temperature may rise excessively, so that there is a problem in that the amount of fresh intake air flowing into cylinders would decrease improperly. For this problem, it may be considered that an EGR cooler to cool the EGR gas and a bypass pipe to allow the EGR gas to bypass the EGR cooler are provided, and two cases in which the EGR gas passes through the EGR cooler to cool the EGR gas or the EGR gas flows bypassing the EGR cooler to keep its high temperature are selectable according to the driving conditions. In this case, however, there is another problem in that the device would become improperly large sized due to providing the EGR cooler and the bypass pipe.